While watching the SES-10 flight, starting a little after T+1:30, I noticed that the direction of the first-stage exhaust plume started to diverge from the alignment of the rocket body.

The divergence angle increased continuously until the plume became too tenuous to clearly determine the direction it was pointing.

After the 2-min mark, the plume appears something like 30 degrees off-line (quite foreshortened, so the actual divergence angle must be smaller).

I thought briefly that it might be deflecting to correct for an engine shutdown, but from 2:02 to 2:14, the 8 separate plumes from outer engines are distinctly visible and angled in the expected directions.

I would expect any deliberate angle-of-attack on ascent to be quite small, and that a gimbaling of the engines that substantial would turn the stack very rapidly. At that speed and altitude, dynamic pressure should still be substantial (perhaps 7-8kPa?).

What explains this observation? Is this just the effect of a strong crosswind, either pushing the plume aside, or forcing the engines to gimbal to correct for the rocket being pushed off trajectory?

@Raze those numbers are not inclination but position in GEO, target inclination is very close to 0° afaik. And parking orbit inclination is somewhere around 28°?
– jkavalikApr 1 '17 at 8:34

3

@Raze that's "Longitude: 67° West (Intended)", not latitude. GEO satellites are all positioned at 0° inclination, but they are all "hanging" over different meridians.
– jkavalikApr 1 '17 at 20:40

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The final position of a geosynchronous orbit has zero effect on the orbital velocity at the end of the ascent. All orbital launches need to transition from vertical ascent to horizontal, and they do it very gradually, with minimal deviations from zero angle of attack. That is not what's going on here.
– Russell BorogoveApr 2 '17 at 3:43

This means that the rocket body is tilted relative to the oncoming airstream. The engine gimbals are nearly in-line with the rocket body; in fact, depending on the distribution of mass within the rocket, they may have to turn very slightly in the opposite direction of the plume deflection to keep the rocket from flipping!

The oncoming airstream at ~1000m/s then deflects the plume parallel to the direction of motion, up to 4.6 degrees from the line of the rocket body. Despite the thin air, dynamic pressure is still quite high due to the high speeds -- ~10kPa at the 2:00 mark, about 1/3 of Max Q -- so it forces the plume into line.

The camera is at the launch site, and the rocket is far downrange, so the view is severely foreshortened, which increases the apparent angle of the deflection from ~4 degrees to ~30 degrees.